Planning Margins Validation for Concurrent Treatment of Prostate and Pelvic Lymph Nodes Under Daily Image Guided Delivery
S Ferjani *, K Stephans , R Tendulkar, P Xia , Dept. of Radiation Oncology Cleveland Clinic Foundation, Cleveland, OH,SU-E-T-649 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To validate prostate planning margins for concurrent treatment of the prostate and pelvic lymph nodes (PLN) under daily kilo-voltage cone beam (KV-CBCT) imaging guidance while applying MLC and iso-center shifting methods to compensate for inter-fractional prostate motion.
Methods and Materials: Ninety-four daily KV-CBCTs from five patients, who received IMRT treatment with a clinical planning margin to the prostate of 8mm/6mm posterior (M(8,6)) and 5mm to the PLN, were selected. Three additional IMRT plans were created for each patient with the prostate planning margins of 6mm/4mm posterior (M(6,4)), 4mm/2mm posterior (M(4,2)) and 2mm uniform margin (M(2,2)). The PLN planning margin remained 5mm. Subsequently, each plan was applied to the daily KV-CBCT using MLC and iso-center shifting methods. Daily D95 of the prostate and D95 of the PLN, and D5 of the rectum and D5 of the bladder were evaluated for adequate planning margins.
Results: For both the MLC and iso-center shifting methods, D95 of the prostate was greater than or equal 97% of the prescription dose in 97.8% (100%), 98.9% (97.9%), 95.8% (97.9%), and 93.7%(96.8%) of 94 fractions, for M(8,6), M(6,4), M(4,2), and M(2,2) respectively. Accordingly, D95 of the PLN was greater than or equal 97% of the prescription dose in 98.9%(100%), 100%(98.9%), 98.9%(98.9%), and 100%(98.9%), for M(8,6), M(6,4), M(4,2), and M(2,2) respectively. For the rectum, D5 exceeded 105% of the original IMRT D5 for the MLC-shift (and iso-center shift) plans in 16%(14.9%), 14.9%(16.4%), 5.4%(12.2%) and 4.3%(12.2%) for these four planning margins respectively. For the bladder, D5 exceeded 105% of the original IMRT D5 for the MLC-shift (and iso-center shift) plans in 0%(1.1%), 6.4% 3.2%), 13.9%(4.3%), and 8.6%(4.3%), respectively.
Conclusion: With 5 mm planning margin to the PLN, both MLC and iso-center shifting methods can reduce the prostate planning margin to 4mm/2mm posterior while achieving adequate targets coverage.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by the United States Army Medical Research and Materiel Command (USAMRMC) (Grant No. W81XWH-080-0358)